As test electronics achieve ever-greater speeds and densities, one significant problem is the removal of the internal heat generated by the tester. In prior generations of automated test equipment, air cooling was sufficient. However, as speeds increased, signal path length has become a critical issue. Minimizing path length has led to miniaturization by a factor of over 1000 in the last 5 years, to the extent that it is no longer practical to air cool current generation automated test equipment. Greater speed compounds the problem, as the heat generation increases with clock speed. Furthermore, customers are demanding higher pin count testers, which increases the difficulty of total power dissipation in the tester.
All of these factors have made liquid cooling more necessary for heat removal from modern test electronics. The current generation of testers must remove about 20 kW of heat from a volume of less than 10 cubic feet. In general, the most reliable methods of liquid cooling seek to isolate the cooling fluid from the electronics of the tester, as opposed to immersion cooling. This is accomplished using various types of heat sink devices, e.g. waterblocks which (sometimes are referred to as ‘cold plates’). The active circuitry may be mounted to a PC board, which in turn may be mounted to a waterblock.
In many machines, circuits are mounted to both sides of a waterblock. This configuration may be used to either minimize space or more fully utilize an expensive component, i.e. the waterblock. In general, the working fluid may be water or some other liquid. Water has the highest cooling performance of the common chosen working fluids, but a variety of considerations may preclude its use.
In general, these waterblocks may be constructed of an easily machined metal with high thermal conductivity. Typically, this metal is either aluminum or copper. Water, or another fluid, may be routed through passages in the metal so as to remove heat. The attachment of the dissipating components to the heat sink, which may be a waterblock. In testers, there may be components attached to the PCB that dissipate a relatively large amount of power over a small area. Other circuit boards may have a multitude of small components that dissipate low amounts of power. The latter configuration may be easily cooled by attachment of the PCBs to the waterblock by a few screws. Due to the relatively low heat dissipation requirements, conduction to the waterblock is effective even with the low average contact pressure exerted by the screws on the board. However, boards with high local heat fluxes pose a challenge. On these boards, certain packages have dissipations of 3-4 W over a 7×7 mm area, or heat fluxes approaching 10 W/cm2. Such high heat fluxes critically require the performance of each part of the system.
Referring to FIG. 1, thermal performance of a system 100 may be analyzed as a function of performance of its parts. Significant thermal resistances for system 100 generally exist from a silicon junction of die 105 to an integrated circuit (IC) case or package 110; significant thermal resistances may exist from case 110 to a top portion 115 of a PCB 120; significant thermal resistances may exist from top portion 115 to a bottom portion 125 of PCB 120; significant thermal resistances may exist from bottom 125 of PCB 120 to a waterblock 130, and significant thermal resistances may exist from waterblock 130 to flowing fluid 135.
A thermal interface material 140 may be used between PCB 120 and waterblock 130. Thermal interface material 140 is provided to achieve good thermal contact between the two rigid surfaces. Generally, the thermal resistance of PCB 120 and the thermal resistance of thermal interface material 140 are the most significant to the overall system. It is the resistance of PCB 120 to waterblock 130 through thermal interface or pad 140 that is the weakest link to achieve efficient cooling in system 100. Although the resistance within PCB 120 is normally greater, that resistance is due to the makeup of PCB 120, and so has little variation from unit to unit.
The resistance from PCB 120 to waterblock 130 is influenced by many factors. The most important factor is the contact pressure of PCB 120 against thermal interface material 140 and waterblock 130. This local contact pressure may vary from several hundred PSI in the immediate vicinity of a screw to less than 10 PSI an inch or more away from a screw. This creates a conflict with screw placement. With more screws, there is better local heat transfer. However, more screws are worse for routing signals through the board. Each screw and its associated keepouts interrupt a significant portion of the routing area for dense high speed boards.
Referring now to FIGS. 2-4, there is shown a system 200 having a heat spreader 205 at least on certain critical circuitry to mitigate the above-identified conflict. Heat spreader 205 generally includes a copper or aluminum plate 205 permanently laminated to back 125 of PCB 120. Plate 205 has two primary functions. First, plate 205 tends to spread out the contact pressure near a screw 400 to a wider area because its greater stiffness than PCB 120. Second, if the thermal contact is poor at one location, plate 205 carries heat sideways to a location of better thermal contact. Since PCB 120 is laminated to the heat spreader 205 under pressure, there is provided a very good thermal and mechanical bond.
Referring now to FIGS. 1-4, heat leaving package 110 is generally carried downward through a series of vias 145. This increases the local effective thermal conductivity. Looking at FIGS. 2-4, heat may easily transfer into heat spreader 205 due to this interface between PCB 120 and heat spreader 205. Heat may be conducted sideways through heat spreader 205 with little resistance due to its metallic nature. Heat flow generally occurs in this manner until a region of contact with sufficient pressure is encountered, such as the region near screw 400, where a thermal path is created for heat to enter waterblock 130. In addition to vias 145, heat transfer from package 110 may be carried through a thermal slug 150 attached to package 110, and through solder 155 between thermal slug 150 and PCB 120.
Although the use of laminated heat spreader 140 is thermally desirable, it may have many disadvantages. Generally, the presence of laminated heat spreader 140 makes PCB 120 almost impossible to troubleshoot or repair. Spreader 140 precludes access to backside 125 of PCB 120, and the thermal mass of spreader 140 may interfere with soldering.